Single-row subquery returns more than one row

I can't seem to wrap my head around this problem I'm having with a query. I need to update all rows in my ps_ntsr_gf_stufile tables with the concatenated values from the ps_classes_tbl table where a.CLASS_NBR = b.CLASS_NBR. I tried to limit it to emplid from the ps_stdnt_enrl table but no luck.